The present invention relates generally to industrial magnetic flowmeters for process measurement and control. Specifically, the invention concerns a magnetic flowmeter sensor having a liner.
A magnetic flowmeter determines a flow rate of a conductive fluid flowing through a conduit in a flowmeter sensor portion of the flowmeter. The magnetic flowmeter measures a voltage generated across the fluid in a direction perpendicular to the fluid flow as the fluid moves through a magnetic field generated within the flowmeter sensor. The voltage is measured between two electrodes in contact with the fluid and positioned on opposite sides of the conduit. The conduit walls must be electrically non-conductive or, if conductive, have an electrically non-conductive liner to keep from shorting out the voltage generated across the fluid flow. If the conduit wall is conductive, the two electrodes must also be electrically insulated from the conduit wall and must penetrate the non-conductive liner to accurately measure the generated voltage.
Polytetrafluoroethylene (PTFE) is a common material choice for a liner in a magnetic flowmeter due to its durability and exceptional resistance to chemical attack. A PTFE liner is fitted to a magnetic flowmeter sensor by inserting a cylindrical PTFE liner into the conduit of the flowmeter sensor, heating the liner, and flaring the liner out at opposite ends of the conduit on to sealing surfaces of the sensor faces. The sensor faces are where the magnetic flowmeter connects into process piping carrying the flow to be measured. However, the flaring process does not leave the liner lying flat against the sealing surfaces. The elastic nature of the PTFE causes the flared liner to spring back, away from the sealing surfaces, leaving a gap between the flared liner and the sealing surfaces. Once installed this gap is closed up by the clamping load of flange bolts connecting the sealing surfaces of the magnetic flowmeter to flanges on the process piping. However, before the flowmeter installation is complete and during the process of handling and installing the magnetic flowmeter, the liner is susceptible to shifting if not handled and installed correctly. Any shifting of the liner can shift the electrodes penetrating the liner and lead to process fluid leaking around the electrodes or result in electrodes shorting against the conduit walls.
One solution to this problem has been to employ adhesives to secure the flared liner to the sealing surfaces. This requires the use of special chemicals capable of overcoming the inherent resistance of PTFE to chemical attack, in order to create a surface suitable for adhesive bonding. Unfortunately, these special chemicals are often hazardous to people, the environment, or both.
Another solution avoids the dangers of the hazardous chemicals by not bonding the liner to the sealing surfaces at all, but relying on clear, specific, and detailed instructions in the proper handling and installation of the magnetic flowmeter. Unfortunately, this solution relies on those who handle and install the magnetic flowmeter to follow such instructions. No matter how excellent the instructions are, it is unlikely that they will be followed in all cases. Thus, a solution is desired that prevents liner shifting in all cases and does not employ chemicals hazardous to people or the environment.